This invention relates to the preparation, storage and transportation of hydrocarbon mixtures. In this field, the mixture is distilled to produce an overhead fraction which is used as the carrier fluid for the heavier fractions. The heavier fractions are carried in the overhead in the form of particles of diameters about 0.05 mm to about 20 mm. These particles are formed by such processes as prilling, extruding or beading and the resulting slurry is pumped in a transporting system that includes a pipeline. Upon pumping, these particles partially dissolve with time and mixing to leave isolated wax crystals.
In order to maximize the percent of each hydrocarbon mixture that can be processed into a transportable slurry, as much overhead as possible must be distilled. The problem in the art has been to determine the maximum amount of overhead that can be obtained from the hydrocarbon without creating a slurry which has too large a pressure drop when pumped. Past methods used the cloud point or pour point to determine the distillation cut point for the overhead. In the pour point methods, the cut point is generally at a fraction whose pour point is about 5.degree. F. to about 10.degree. F. below the average temperature of the transporting system.
Experimental data has shown that the use of the cloud point or pour point to determine the distillation cut point is unreliable. In some cases, a low viscosity overhead with a pour point higher than the slurry pumping temperature contributed little to the slurry pressure drop whereas a lower pour point overhead (10.degree. F. below the slurry pumping temperature) contributed significantly to the pressure drop within the pipeline. In particular, methods that use the cloud point or pour point to determine the maximum amount of overhead that can be taken do not consider the resulting amount of wax crystals in the overhead due to the distillation process used. These wax crystals are much smaller than the congealed particles of the heavy fractions in the slurry and their presence in the overhead greatly increases the pressure drop. After the congealed particles disintegrate leaving isolated wax crystals, some wax crystals from the overhead are still detrimental to the slurry pumpability.
Processes such as flash distillation can have a large overlap in the cuts and can result in overheads with a great number of wax crystals. The wax crystals are from fractions that boil within the range of the heavy cuts and crystallize in the overhead at temperatures above the slurry transporting temperature. Additives do not affect the amount of wax crystals in the overhead but can modify the crystal structure of the wax which crystallizes in the presence of these additives and be beneficial in some, but not all cases. Regardless of the cause, these wax crystals are more economically conveyed in the congealed particles of the heavy fractions than in the overhead and should be minimized in the overhead.
This invention provides a method to determine the optimum overhead properties and to monitor and control the distillation of a hydrocarbon mixture in order to produce as much economically pumpable overhead as possible. It also increases the efficiency of transportation of hydrocarbon slurry systems by the use of continuous analysis of the feed.